2016, Fazle Rabbi, S M, Daniel, Heiko, Lockwood, Peter V, Macdonald, C, Pereg, Lily, Tighe, Matthew, Wilson, Brian, Young, Iain

Aggregates play a key role in protecting soil organic carbon (SOC) from microbial decomposition. The objectives of this study were to investigate the influence of pore geometry on the organic carbon decomposition rate and bacterial diversity in both macro- (250-2000 μm) and micro-aggregates (53-250 μm) using field samples. Four sites of contrasting land use on Alfisols (i.e. native pasture, crop/pasture rotation, woodland) were investigated. 3D Pore geometry of the micro-aggregates and macro-aggregates were examined by X-ray computed tomography (μCT). The occluded particulate organic carbon (oPOC) of aggregates was measured by size and density fractionation methods. Microaggregates had 54% less μCT observed porosity but 64% more oPOC compared with macro-aggregates. In addition, the pore connectivity in micro-aggregates was lower than macro-aggregates. Despite both lower μCT observed porosity and pore connectivity in micro-aggregates, the organic carbon decomposition rate constant (Ksoc) was similar in both aggregate size ranges. Structural equation modelling showed a strong positive relationship of the concentration of oPOC with bacterial diversity in aggregates. We use these findings to propose a conceptual model that illustrates the dynamic links between substrate, bacterial diversity, and pore geometry that suggests a structural explanation for differences in bacterial diversity across aggregate sizes.

2012, Ghosh, Subhadip, Yeo, D, Wilson, Brian, Ow, L F

Urban soils are a key component of the urban ecosystem but little research has considered their quality and management. The use of char or partially combusted char products as a soil amendment is becoming popular worldwide because of perceived benefits to fertility and the potential for increasing carbon sequestration. In this study, we assessed the effect of applying coarse and fine char material on the quality of four different types of soil-based root-zone mixes typically used for turfgrass and general landscaping in Singapore: clay loam soil, approved soil mix (ASM, 3 soil:2 compost:1 sand), 50:50 (sand/soil) and 75:25 (sand/soil). Char briquettes made from sawdust were mixed thoroughly at rates of 25, 50 and 75% by volume with the soil mixes. Results showed that addition of char (both coarse and fine) significantly enhanced the carbon content of the mixes, with the largest increase being associated with the 50% and 75% additions. Soil nutrients (total N, extractable P, K, Ca and Mg) and mean weight diameter of aggregates were also significantly increased following the application of char. The clay loam and the 50:50 and 75:25 soil mixes were more responsive to the addition of char than was ASM.

2011, Munoz-Robles, Carlos, Reid, Nicholas, Tighe, Matthew, Briggs, Sue, Wilson, Brian

Resource retention is an important component of landscape function in semi-arid environments, with patches in the landscape serving as sink zones, capturing runoff, sediments and nutrients from inter-patches or source areas. The overall aim of this study was to compare the hydrological and erosional responses of patches and inter-patches using small-scale (1 m²) rainfall simulation in four vegetation states comprising woody encroachment (trees and shrubs > 1200 stems ha⁻¹), recent pasture (< 23 years of age), long-established pasture (50–100 years of age) and open woodland in relation to ground cover, in semi-arid south-eastern Australia. Hydrological and erosional responses differed consistently between patches and inter-patches within each vegetation state. Inter-patches (mean ground cover = 23.5% ± 2.7% SE) had the least desirable hydrological and erosional responses, and produced the highest amounts of runoff and sediment, followed in decreasing order by medium vegetated patches (mean ground cover = 54.8% ± 3.1% SE) and well vegetated patches (mean ground cover = 77.3% ± 3.1% SE). However, when hydrological and erosional responses from the same patch type and inter-patches were compared between vegetation states, two differences were found. Well vegetated patches in woody encroachment produced higher runoff than other well vegetated patches, and inter-patches in recent pasture had higher sediment concentration and production than inter-patches in other vegetation states. Total ground cover in the patch types was negatively correlated with runoff and sediment production, and with sediment production in inter-patches. Cryptogam cover in well vegetated patches was associated with higher runoff and sediment production, but cryptogam cover in inter-patches was associated with lower sediment concentration and production. Herbaceous cover and litter in medium vegetated patches were associated with lower runoff and sediment production. Our results indicate that patches and inter-patches are functional units from an eco-hydrological perspective within this semi-arid region and they influence soil hydrological and erosional characteristics irrespective of vegetation state.

2015, Ghosh, Subhadip, Ow, L Fern, Wilson, Brian

Research relating to the use of organic amendments on soils has focused largely on agricultural soils, and there is a lack of information worldwide on their efficacy as amendments for urban soil management, especially in tropical urban environments. A pot experiment was conducted to assess the influence of biochar and organic compost on urban soil properties and on tree growth performance in Singapore. Biochar and compost were mixed with topsoil in different proportions, and two urban tree species commonly grown in Singapore ('Samanea saman' and 'Suregada multiflora') were used. There were significant additional height increments for both the tree species following application of biochar. 'S. saman' exhibited greater stem elongation compared with 'S. multiflora' in response to organic amendments. A significantly higher foliar N content was found in both tree species in biocharamended treatments along with significant increases in P and K. Increases in soil nutrient concentrations were also observed in combined biochar-compost treatments for both species. Combined compost and biochar had the strongest effects on soils and growth of the two urban tree species examined and applications containing biochar resulted in the most significant soil improvements.

2012, Ghosh, Subhadip, Wilson, Brian, Ghoshal, Subrata, Senapati, Nimai, Mandal, Biswapati

Soil organic carbon is considered to be of central importance in maintaining soil quality. We assessed the effects of a range of commonly applied organic and inorganic amendments on soil quality in a rice-wheat cropping system in the Indo-Gangetic plains of eastern India and evaluated the carbon sequestration potential of such management approaches using a 25 year old long-term fertility experiment. Results showed that there were significant increases in soil nutrient availability with the application of farm yard manure (FYM @ 7.5 t ha⁻¹), paddy straw (PS @ 10 t ha⁻¹) and green manure (GM @ 8 t ha⁻¹) along with inorganic fertilizer. Both microbial biomass C and mineralizable C increased following the addition of the organic inputs. Continuous cultivation, without application of organic inputs, significantly depleted total C content (by 39-43%) compared with treatments involving the addition of organic amendments. A significant increase in the non-labile C fraction resulted from both organic and inorganic amendments, but only 26, 18 and 6% of the C applied through FYM, PS and GM, respectively was sequestered in soils. A significant increase in yield of kharif rice was observed as a result of the addition of these organic amendments.

2009, McHenry, Melinda Therese, Wilson, Brian, Guppy, Christopher, Sindel, Brian, Lockwood, Peter

Woody vegetation thickening and encroachment is a global issue of both environmental and economic significance. On the North Western Slopes and Plains of New South Wales, Australia, the indigenous conifer Callitris glaucophylla Joy Thomps. & L.A.S. Johnson (White Cypress Pine) regenerates prolifically, especially in previously cleared agricultural landscapes. The species is recognised in state legislation as 'Invasive Native Scrub' and strategies to manage the species in a manner that improves or maintains on-farm environmental condition are now required by law. The primary objective of this study was to address some of the present knowledge gaps that exist regarding the role and function of the species in a range of configurations in agricultural landscapes of the region. A secondary objective was to draw upon data resulting from this project to suggest optimum management strategies for the species under a variety of scenarios. The investigative approach taken in this study employed a combination of field surveys, field experiments and glasshouse trial, each of which were concerned with describing and/or manipulating the effects of the species on soil and vegetation parameters. As a prelude to this project, a small pilot study was used to identify soil and vegetation parameters most likely to respond to changes in the management of the species. This study used a paired site approach to revisit sites thinned or maintained as controls since 2000 by Greening Australia and compared them with adjacent cleared areas as yet un-colonised by C. glaucophylla. In general, soil and vegetation parameters varied due to site and land-use or their interaction. There was a general trend of increased litter cover from cleared zones through to control zones across all sites, whilst vegetative groundcover decreased with increasing tree cover. The diversity of vegetative groups was linked to site effects. Soil C, N, S and ext. P were generally more abundant in the mineral soil of treed areas and were significantly less abundant in cleared zones, although soil pH was significantly more acid in control plots than thinned or cleared areas. The study also identified two possible economic benefits of the species for landholders. The growth of trees in thinned plots in the five years since management suggested that it may have farm-forestry potential. This growth, in conjunction with the carbon content of all other site components resulted in an estimated total site carbon that was much higher in thinned and control plots than cleared areas and hence the carbon trading potential for the species in a agricultural setting was also suggested. The results of this study indicated that, for the parameters studied, thinning maintained environmental condition.

2019-02-27, Tessema, Bezaye Gorfu, Wilson, Brian, Daniel, Heiko

Globally, soil organic carbon (SOC) has declined as a result of human induced disturbance with negative effects on production and productivity. Maintaining SOC has the combined effect of contributing to climate change mitigation efforts and agro-ecosystem functioning in addition to its potential for sustaining soil health. A primary source that can contribute to soil carbon (C) sequestration is plant biomass, and an important component of this is the biomass found belowground. SOC sequestration using plant species with high photosynthetic efficiency, deep roots and high biomass production therefore has considerable potential for soil carbon storage. Perennial tropical grasses, particularly those with deep root systems, are therefore likely to contribute significantly to SOC and the introduction of perennial tropical grasses could potentially contribute large quantities of C through the soil profile and facilitate SOC sequestration. A range of tropical pasture species have been investigated for their SOC storage potential, but vetiver grass, given its extensive use globally and its large biomass production, has considerable, as yet unquantified, potential for long term C storage. The main aim of this research was to examine the SOC quantity, nature and distribution in soils under vetiver. Specifically, the work 1) examined SOC content, stock and profile distribution under vetiver; 2) determined the quantity of SOC attributable to vetiver (C₄-C) compared with soil dominated by previous C₃ carbon; 3) examined the above- and below-ground vetiver biomass production and the relative rate of decomposition, and 4) determined the allocation of soil C under vetiver to its component fractions (POC, HOC and ROC) differentiated on the basis of particle size and chemical composition.

A series of research questions were examined under this PhD research work: In chapter 3 undisturbed soil core samples were collected to 1.0 m soil depth from Gunnedah, Australia to determine the soil carbon content and depth distribution down the soil profile under vetiver compared with native and tropical pastures and cropland soil. The result showed a larger TOC stock under vetiver (123 Mg ha^-1) compared with tropical pasture (93 Mg ha^-1) and cropping soils (78 Mg ha^-1) while vetiver and native pastures (111 Mg ha^-1) showed no significant difference in TOC stocks. For all plant types, a decrease in SOC content was observed with increasing soil depth but a larger stock of C was found under vetiver at almost all depths through the soil profile compared with cropping soils, but on an annual basis, not much more than other tropical grasses. Soils under vetiver had higher (less negative) δ¹³C compared with native, tropical pastures and cropping soils. This was particularly true in the surface soil layers but persisted to some degree through the whole soil profile. Both litter and roots probably contributed to the additional C stock by vetiver (43.5%) and results indicated a significant C turnover through the whole soil profile resulting in a modest net accumulation of soil C.

In chapter 4 the impact of vetiver grass on carbon sequestration and its SOC input and the quantity of SOC attributable to vetiver (C₄ carbon) compared with soil dominated by pre-existing (C₃) Carbon determined. Undisturbed soil core samples were collected to 1.0 m soil depth from Southwest Ethiopia. The result showed a larger TOC stock under vetiver (mean 262 Mg C ha^-1) compared with coffee (mean 178 Mg C ha^-1), particularly, at the surface soil layers and decline was observed with increasing soil depth between plant types. Low δ¹³C (more negative) values were recorded at the soil surface layers increasing with increasing soil depth for both vetiver and coffee. However, the δ¹³C values were significantly higher (less negative) under vetiver in comparison with coffee, particularly at the surface soil layers which suggests a continuous new C addition and a significant C turnover in the soil system.

In chapter 5 vetiver plant material was therefore grown under a glasshouse condition for biomass production assessment and subsequently incubated to determine the relative decomposition rate between the above- and below-ground vetiver biomass in different soil types. Vetiver showed a high biomass production (268 Mg ha^-1 of fresh and 120.2 Mg ha^-1 of dry biomass) potential and the shoot to root biomass ratio was determined to be 1.49 and 1.28, for the fresh and dry biomass, respectively.

In chapter 6 the amount of allocation of soil carbon to particulate, humus and resistant fractions differentiated based on particle size and chemical composition. The stocks of soil C fractions indicated significant variations which changes from the labile POM to the HOM across site and vegetation types. Hence, the dominant C fraction was HOC (58%) for vetiver and all vegetation types. The ratio of POC to HOC stocks was also very low indicating the lesser vulnerability of C because of the high proportion of HOC component fraction given its less labile nature which could help the carbon stay in the soil for longer time and changes quite slowly.

Despite the continuous new C addition under vetiver the significant soil C turnover could be due to the more rapid decomposition of the root material than the shoot which could have been impacted by the lower C:N ratio of the root compared with the shoot. Hence, promoting the use of vetiver, particularly due to its potential to produce a large biomass, is a promising strategy to enhance soil C storage. Hence, growing vetiver has the potential for high rate of C accumulation because this grass is building up the more stable HOC fraction which is less vulnerable to change and to use this in the C accounting program can be feasible. This study investigated that vetiver due to its fast growth, large biomass production (both above- and below-ground) potential and extensive use has considerable potential for C sequestration, particularly on C depleted soils. In conclusion, in this work it has been demonstrated that vetiver grass has an important role in storing large TOC stock, has the potential to add new carbon despite high rates of turnover; produce high biomass and have high root to shoot decomposition which might be a reason for high turnover rates and larger organic carbon accumulation in the more resistant (hemic organic carbon fraction) carbon pool throughout the 1.0 m soil profile and has considerable potential for both restoration of soil health and for storing additional soil carbon to offset greenhouse gas emissions.

2010, Munoz-Robles, Carlos, Reid, Nicholas, Frazier, Paul, Tighe, Matthew, Briggs, Sue, Wilson, Brian

Gullies initiate when the equilibrium within a drainage line is upset by increased discharge or decreased soil resistance to detachment and transport of soil particles, and is triggered and accelerated by human-induced changes to vegetation cover. Woody encroachment is the increase in density, cover, extent and biomass of woody plants, and can result in low ground cover, which increases the potential of soil erosion. This paper reports the relationships between gully erosion and site characteristics such as topography, vegetation cover and road infrastructure in a 120-km² study area of semi-arid woodland subject to woody encroachment in south-eastern Australia. This study characterised gully morphology and activity, identified the factors related to gully volume, and estimated the topographic thresholds for sub-catchments subject to woody encroachment. Firstly, relationships between gully volume, activity, topography, vegetation cover and road infrastructure were explored. Secondly, site characteristics of 32 sub-catchments with and without gullies were compared. Finally, the topographic thresholds for unstable and stable sub-catchments with and without gullies, respectively, were estimated using the slope – drainage area relationship (S = aA⁻ᵇ). All gullies were active and high gully volume was related to large sub-catchments, long gullies and short distances to roads. Stable sub-catchments (without gullies) had gentler slopes, higher foliage projective cover and ground cover than unstable sub-catchments (with gullies). When data for sub-catchments with and without gullies were combined, the main factors related to gully volume were drainage area at the gully or stream heads and their local slope, and ground cover. The mean topographic threshold for gully development provided insights into the drainage area and slope conditions needed for gully development. Gully erosion in the study area is most likely the result of interactions among topography, vegetation and human-made structures (roads) in space and time. The results of the study advance our understanding of gully erosion dynamics in areas of woody encroachment, and can be used to identify areas susceptible to gullying.

2013, Munoz-Robles, Carlos, Tighe, Matthew, Reid, Nick, Frazier, Paul, Briggs, Sue V, Wilson, Brian

Management of woody encroachment and pasture to reduce runoff and sediment production is important in semi-arid areas. However, the study of relationships between vegetation and surface hydrology at hillslope scale is difficult because of cost and time constraints. Up-scaling eco-hydrological responses measured at fine scale can overcome these constraints and provide insights into runoff and erosion at scales relevant to management. In this study, runoff and sediment production were modelled on two adjacent hillslopes, one with woody encroachment (3500 stems ha⁻¹) and the other a volunteer pasture cultivated to oats 18 months previously. Spatial modelling was undertaken to integrate small-plot (1 m²) rainfall simulation, slope and the spatial distribution of ground cover. The estimates of runoff and sediment production in the woody hillslope were considerably lower than in the pasture hillslope in both years of the study. Runoff and sediment production in the woody hillslope were similar in consecutive years, whereas the estimates of runoff and sediment production in the pasture hillslope were lower in the second year as a result of the establishment of a water spreading system of contour banks. The results showed the importance of measuring patchiness and connectivity of runoff source areas for runoff and sediment production. The spatial modelling approach allowed a description of fine-scale, surface eco-hydrological interactions on hillslopes, based on high resolution spatial data and experimental fine-scale rainfall simulations. A similar modelling approach could be used to explore runoff and sediment production resulting from varying management of semi-arid lands.

2010, Munoz-Robles, Carlos, Tighe, Matthew, Reid, Nicholas, Briggs, Sue, Wilson, Brian

Resource retention is an important component of landscape function in semi-arid environments, with patches in the landscape serving as sink zones that capture runoff, sediments and nutrients sourced from inter-patch areas. The purpose of this rainfall simulation study was to compare hydrological and erosional responses in patches and inter-patches in woody encroachment areas (trees and shrubs >700 stems ha⁻¹) in semi-arid south-eastern Australia. Ground cover, hydrological and erosional responses differed consistently between patches and inter-patches. Inter-patches had low ground cover and produced more runoff and sediment than patches with medium to high ground cover. Patches displayed delayed initiation of runoff and a deeper soil wetting front. Litter, cryptogam cover and surface sand content were the main variables controlling average runoff rate, sediment concentration and total sediment production. The results indicated that patches and inter-patches are different functional units from an eco-hydrological perspective and influence the hydrologic and erosional characteristics of woody encroached areas.